1. Field of the Invention
The present invention relates to a magnetoresistive (MR) sensor with a soft adjacent layer (SAL) having high magnetization, high resistivity, low intrinsic anisotropy and near zero magnetostriction, wherein the sensor is an anisotropic MR sensor or a spin valve sensor.
2. Description of the Related Art
A well-known magnetoresistive (MR) head configuration employs an anisotropic magnetoresistive (AMR) sensor sandwiched between a first gap layer and a second gap layer. These gap layers are, in turn, sandwiched between a first shield layer and a second shield layer. An AMR sensor typically includes a ferromagnetic MR stripe and a layer of soft magnetic material spaced from the MR stripe by a spacer layer of insulative material. The layer of soft magnetic material, referred to as the “soft adjacent layer” (SAL), produces a transverse bias in the MR stripe. In this regard, an MR stripe has a magnetic moment which is biased at an angle to an air bearing surface (ABS) of the head by the SAL. Other layers in an AMR sensor may include a seedlayer for the MR stripe and a capping layer. First and second leads are connected to the MR sensor for conducting a sense current through the sensor. When the MR stripe is in a magnetic field emanating from a moving magnetic medium, such as a rotating magnetic disk, the resistance of the MR stripe changes according to the function cos2θ, where θ is the angle of the magnetic moment to the ABS. With a constant sense current, the resistance change produces a voltage change that is detected as a readback signal.
The bias produced by the SAL depends upon the sense current. When the sense current is conducted through the MR stripe, the MR stripe produces a sense current field that is coupled to the SAL. This causes the SAL to produce a demagnetization (demag) field or stray field that is coupled to the MR stripe. The field coupled to the MR stripe rotates the magnetic moment of the MR stripe from a position parallel to the ABS to an angle to the ABS. At a rotation of 45°, the MR stripe is biased to a generally linear portion of its response curve. Other schemes have been proposed for biasing the MR stripe, however, the SAL scheme is the most common because it does not require a separate current source. However, it is known that a SAL shunts a portion of the sense current between the first and second leads, which are connected to all of the layers of the AMR sensor. Accordingly, it is important that a SAL material be selected that has a high resistance without sacrificing other desirable properties for a SAL.
The desirable properties for a SAL are as follows:                (1) high resistivity;        (2) high magnetization;        (3) soft magnetic properties;        (4) near zero magnetostriction;        (5) thermal stability;        (6) corrosion resistant;        (7) low MR coefficient; and        (8) low coercivity.        
High resistivity has already been explained. High SAL magnetization desirably maximizes the bias of the MR stripe. Soft magnetic properties are important because the material can be saturated with a small applied field (HK). An HK of less than 15 Oe is desirable. Near zero magnetostriction is important to minimize a stress-induced anisotropic field. The stress-induced anisotropic field either adds to or subtracts from the intrinsic anisotropic field of the SAL, depending upon whether magnetostriction is positive or negative. The magnitude of a stress-induced anisotropic field is unpredictable and can increase dramatically after lapping the ABS of the head.
It is important that the SAL material be thermally stable during high temperatures as may be reached during fabrication of the head or during operation. It is important that the material not change from an amorphous state to a crystalline state during these high temperatures.
Since an edge of the SAL is exposed at the ABS it is important that the material selected for the SAL be corrosion resistant. Corrosion changes the properties of the material. The resistance to corrosion is also important for enhancing the thermal stability of the head. If the material is corrosion resistant the material is less likely to change its composition by reacting with adjacent thin film layers. As is known, such a change in composition can dramatically change the properties of the SAL material.
A low MR coefficient is important so that the SAL does not compete with the MR coefficient of the MR stripe. It is also desirable that the magnetization of the material is uniform and that its magnetic moment can be smoothly switched upon the application of positive and negative applied fields.
A typical material employed for the SAL is NiFeCr. NiFeCr exhibits the above properties to an acceptable degree. However, if one or more of these properties can be improved, the performance of the head can be dramatically improved. For instance, if the magnetization of the SAL material can be improved then the resistance of the SAL can be improved. Assume that a predetermined bias field is required to bias the MR stripe and that material A with a predetermined thickness will supply the required bias field. Assume that material B has a higher magnetization than material A and has approximately the same resistivity as material A. Then a thinner layer of material B can be employed for supplying the required bias field. Furthermore, the resistance of the SAL will be higher with material B than with material A. Therefore, use of material B will reduce shunting of the sense current through the SAL. Manifestly, careful selection of a soft magnetic material may result in improvement in SAL performance in more than one respect.
U.S. Pat. No. 4,994,320 teaches the use of CoZr based materials for a SAL. The patent states that CoZr exhibits instability of the anisotropy field when it is made thin enough (200-1,000 Å) to be employed as a SAL. The patent also suggests substituting Hf or Nb for Zr. In order to correct the instabilities, the patent teaches depositing a first cladding layer of SiO2, depositing by sputtering the cobalt alloy along with silicon on the first cladding layer and then depositing a second cladding layer of SiO2 on top of the sputtered layer. While not addressed in the patent it is believed that the instability of the anisotropy field of CoZr is due to poor magnetostriction and poor corrosion resistance. High magnetostriction is inherent in CoZr and will result in an unstable anisotropy field. Poor corrosion resistance will permit the material to corrode or react with adjacent layers at high temperatures which will change the anisotropy field.
A published Japanese patent application No. 5-36033 teaches the use of thin (2-100 Å) CoHfNb layers laminated with a thin (2-100A) nonmagnetic metal layer for use in magnetic recording head. The nonmagnetic metal layer is sandwiched between the CoHfNb layers. Although not addressed in the patent the laminated layers are apparently employed to produce a giant magnetoresistance (GMR) effect. This application is non-analogous to the function of a SAL. CoHfNb has also been employed for write poles in write heads and as shield layers in MR read heads. These applications are also non-analogous to the function of a SAL.
In IEEE Transactions on Magnetic, Vol 26, No. 6 dated November 1990, Yamada et al. teach the use of CoZrMo as a SAL for an AMR sensor. Mo was employed in the alloy to decrease the uniaxial anisotropy (HK) so that the SAL could be saturated by the sense current field from the MR stripe. Unfortunately, Mo seriously degrades the magnetization of Co. Further, in IEEE Translation Journal on Magnetics in Japan, Vol 2, No. 4 dated April 1993 Yamada reports degradation of CoZrMo when subjected to heat during fabrication or operation.
An IBM development employs a SAL in a spin valve sensor. A spin valve sensor includes a non-magnetic conductive layer sandwiched between a pinned layer and a free layer. The pinned layer has its magnetic moment pinned in a predetermined direction, such as perpendicular to the ABS, while the free layer has its magnetic moment aligned along an easy axis parallel to the ABS, but free to rotate when subjected to a magnetic field. The magnetic moment of the pinned layer is typically pinned by exchange coupling with an anti-ferromagnetic layer. When the magnetic moments of the free and pinned layers are parallel, the resistance of the spin valve is at a minimum; when the magnetic moments of the free and pinned layers are anti-parallel, the resistance of the spin valve is at a maximum. Accordingly, the relative rotation of the magnetic moments of the pinned and free layers changes the resistance of the spin valve. These changes produce voltage changes that are detected by circuitry to produce a readback signal. The IBM development teaches the use of a SAL as a “keeper” for minimizing a demagnetization (demag) field from the pinned layer. An insulative or conductor spacer layer is sandwiched between the free layer and the SAL. When sense current is conducted through the spin valve sensor a sense current field from the free layer is induced on the pinned layer.
During normal operation, the temperature of the spin valve sensor may rise. Unfortunately, as the temperature increases the magnitude of the exchange pinning field falls. Without the SAL as a keeper, the demag field will, at some temperature, exceed the exchange pinning field causing the magnetic moment of the pinned layer to become disoriented. With the SAL as a keeper, the demag field from the pinned layer is attracted by the SAL and reduced virtually to zero. With this arrangement, disorientation of the magnetic moment of the pinned layer is expected to occur at a much higher temperature.
The aforementioned properties which are desirable for a SAL for an AMR sensor also apply to a SAL for a spin valve.